Although different from the classic private messages system, I found this extension as a really interesting new way to communicate privately or in groups. Choosing tags in private discussions is great when I want to discuss some specific topics, but it would be great if choosing tags is not a mandatory option. There are moments when you want to talk or ask someone something else.
I think that it would be great if this extension can offer as an option to choose one tag as a default one for private discussions. The default tag would be selected if there are any recipients chosen and not tags chosen. That tag than would be on top of others, as I tried to picture below.
This is a relatively simple solution that wouldn't change the nature of this extension (for which I understand you developers don't want to be classic private messages system) but would please all of us that are looking for something more personal.